In a liquid crystal matrix display, a multiplicity of active elements, typically thin film field effect transistors, are each associated with a volume of liquid crystal that is the pixel element of the display. These active elements are fabricated on one of the two panels enclosing the liquid crystal material. When the active elements and/or associated circuits are defective, the associated pixel cannot be controlled and the display is correspondingly compromised. An individual pixel of the display is typically addressed by coupling the active elements in a grid consisting of rows and columns. The testing of the individual pixels has involved application of an activation signal to the control grid (gate terminal of a field effect transistor) of the active (row) element associated with the pixel under test and the application of a voltage designed to alter the optical properties of the liquid crystal volume associated with the (column) pixel to the input terminal (source terminal of a field effect transistor) of the active device. The display could be activated with an image that, when accurately represented by the liquid crystal display, would provide an opaque image. By proper illumination, the pixels that were not rendered opaque by the activating image could be determined by visual inspection.
The testing procedure of the related art suffered from several weaknesses. As the pixels have become smaller in order to achieve higher resolution images the ability to detect a malfunctioning pixel has become more difficult and time consuming. The principal disadvantage of the testing procedure of the prior art methods in addition to not lending itself to automated procedures, was that the testing could not be performed until the display was completely assembled, thereby minimizing the opportunity for repair of the defective elements.
A need has, therefore, been felt for apparatus and a related procedure that can test the active elements associated with each pixel of a liquid crystal display which is suitable for automation techniques. In addition, a testing technique is needed that can be performed after fabrication of the active elements, but before final assembly of the display.